Increasing demand for more efficient lighting sources has led many industries away from incandescent, arc and induction light sources and into solid state lighting. Solid state lighting has primarily been represented by light emitting diodes due to their long duty cycles and high rate of conversion of electrical energy into light (at about 80% efficiency). Light-emitting diode (LED) light sources are not, however, without their drawbacks. First, LEDs are narrow-band emitters, typically covering a band of some 10 nm at half height of the output curve. This makes emission of full spectrum white light from an LED-only illuminator highly impractical, as doing so would require tens of different-wavelength emitters. Consequently, lighting manufacturers have used combinations of LEDs and phosphors that are excited by the center wavelength of a given color LED to approximate white light. This approach also has its drawbacks, in that the spectral output profile is characterized by a spike at the center of the LED's emission band surrounded by a roughly 10 nm half-height curve, then a broad but lower amplitude band (as compared with the LED output) from the secondary phosphor emission, typically representing one-quarter to one-third of the visible spectrum, which then trails off to near zero emission at the lower visible frequencies, resulting in an approximate “white light” output that is deficient in the green and red bands. So-called “warm white” LED/phosphor chips shift the phosphor output lower in the frequency band, but still exhibit broad gaps of little or no emission in at least two spectral bands.
LEDs also present significant issues in controllability of output light due to the planar configuration of the emitter. Etendue imposes strict requirements upon the size of a collimator necessary to attain a given collimation angle for the collected beam from the emitter. With current available LED output coupled with practical luminaire size constraints, beam collimation is typically limited to no tighter than 8° without significant loss of optical efficiency. This makes LEDs relatively inefficient sources for imaging optical systems and especially for collimating systems such as searchlights and spots.
The solid state alternatives to LEDs are lasers. In the past, LEDs have held a significant cost and efficiency advantage over lasers for general illumination applications. While LEDs typically convert approximately 80% of electrical energy consumed into light, older lasers typically converted electricity at a rate of only about 20%. Lasers were also difficult to cool and also exhibit narrow band emission patterns similar to those of LEDs.
Advances in laser technology have resulted in lasers capable of conversion of electrical energy to light at closer to 60%, comparing favorably with the 80% conversion rate for LEDs. It has also resulted in dramatically less expensive laser modules with significantly reduced cooling requirements similar to those for LEDs and have service lives that compare favorably with high-output LEDs. And while still more expensive and less electrically efficient than LEDs, laser light has the advantage of being coherent, and not dispersive as is the light produced by planar LED emitters. This allows for vastly better control of output light, resulting in optical efficiencies that may be several times better than those for LEDs in highly collimated beams, overcoming cost and electrical conversion efficiency deficiencies relative to LEDs.
In addition to the coherent nature of coherent light sources making higher degrees of collimation of output light possible, it also lends itself to combination of multiple emitters of differing chromaticity into a single beam via readily available optical combiners. While LEDs may also be combined via combiners such as dichroic “x-cubes,” efficiency is compromised by the limited ability to collimate the output from the individual emitters. This results in significant portions of the beams from these emitters striking dichroic elements off-axis, thereby limiting the efficiency of the reflectivity of the dichroic elements and resulting in light loss. The coherent light produced by lasers does not suffer this light loss.
The value of combination of heterogeneous emitters into a single beam is the capability better to approximate full-spectrum light. This is especially true if multiple phosphors excited at different wavelengths and emitting in different visible light bands may be incorporated into the system. Innumerable lighting applications require a high degree of collimation from light sources. These include searchlights, theatrical fixtures, spotlights and cinema lighting. Likewise, innumerable lighting applications require full-spectrum white light (or at least a reasonable approximation thereof). These applications include lighting for television, cinema production and theater and art gallery lighting, where accurate representation of illuminated pigments or full representation of the spectrum for excitement of CCDs is necessary.